1. Field of the Invention
This invention relates to nuclear power systems, and more particularly to grid structures for nuclear reactor fuel elements, and the like.
2. Description of the Prior Art
In order to produce useful nuclear power, a suitable concentration of fissionable material must be assembled in a reactor core under proper conditions. This material, of which the uranium isotope U.sup.235 is typical, generates power through sustained fission processes. In the usual fission events, neutrons are absorbed in the U.sup.235 nuclei. These absorptions cause the nuclei, in general, to release energy and to disintegrate into the nuclei of lighter elements. If the processes are to continue, the fissioning nuclei also must produce a new generation of neutrons in an abundance that is sufficient to initiate an equal number of new fissions.
There are many formidable environmental, physical, and economic difficulties that must be overcome in order to build a commercially acceptable nuclear reactor while maintaining this essential neutron balance. For example, in a power reactor that transfers the fission process energy to a flowing stream of pressurized water, the fissionable material is loaded into hollow tubes that are referred to as "fuel rods".
For ease of shipment, installation, and removal, as well as to enhance the structural integrity of the core, these rods usually are grouped together in sub-assemblies that are called "fuel elements". A typical commercial reactor fuel element, for instance, may have an array of more than two hundred of these individual fuel rods.
In an assembled fuel element, the rods usually are separated from each other to provide spaces for coolant water flow in order to remove the heat that is generated in the core. This rod arrangement, moreover, when viewed in cross section, is generally square.
The individual rods are held in their relative positions by means of cellular grid structures that are formed from interlocking plates. The grids, moreover, are spaced from each other at intervals along the length of the array of fuel rods. Each of these fuel rods in the array are lodged in respective cells within each of the grids. Within each of the cells, the respective fuel rods are restrained, or engaged by detents or "stops" that protrude from the plates that form the grid and press against the respective fuel rod surfaces.
These grids, however, conflict with other desirable features of a well-designed reactor core. For instance, to make the most efficient use of the fissionable material charge within a reactor core, pressure losses in the core coolant should be minimized. Further in this regard, because the fuel elements ordinarily are nested closely together in a reactor pressure vessel, it is customary to provide tongues on the bands that form the peripheries of the grid structures. These tongues have portions that jut out from the bands to guide the fuel elements as they are being inserted into or being withdrawn from the reactor pressure vessel. Although these tongues perform a useful function, they nevertheless tend to obstruct the flow path and thereby increase the coolant pressure losses.
These tongues also impose an adverse influence on the neutron balance within the reactor core. In this regard, it should be noted that almost all non-fissionable materials within a reactor core tend to act as "poisons" that absorb neutrons without producing a corresponding generation of neutrons to sustain the fission process. Thus, the tongues increase these parasitical neutron losses within the reactor and thereby decrease the "life" of the core.
The clear desirability of reducing these losses, however, is superseded by the need to insure smooth insertion and withdrawal of the fuel elements from the reactor vessel. This need is further emphasized if it is recognized that during withdrawal, the individual fuel elements probably will have to be handled with remotely operated tools because of dangerously high radioactivity levels. If, for example, two radioactive fuel elements should lock together during withdrawal from the core, the problem of disengaging these elements from each other to complete the withdrawal through some sort of remote manipulation can be time consuming, expensive and possibly dangerous.
One suggested technique proposes welding the grid plates to the fuel rods and eliminating the bands that circumscribe the grid structure. On one side, the ends of the individual plates protrude beyond the fuel element framework and the corners of these protruding plates are tapered. The welded structure, however, can lead to manufacturing, processing and quality control difficulties.
Accordingly, there is a need for some fuel element technique that will aid reactor core assembly and reduce coolant pressure losses and parasitical neutron absorption.